netdev - Re: [PATCH net-next v2 1/2] net/ipv6: insert a f6i to a GC list only if the f6i is in a fib6

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Date: Fri, 8 Dec 2023 15:19:28 -0800
From: Kui-Feng Lee <sinquersw@...il.com>
To: David Ahern <dsahern@...nel.org>, thinker.li@...il.com,
 netdev@...r.kernel.org, martin.lau@...ux.dev, kernel-team@...a.com,
 davem@...emloft.net, kuba@...nel.org, pabeni@...hat.com, edumazet@...gle.com
Cc: kuifeng@...a.com, syzbot+c15aa445274af8674f41@...kaller.appspotmail.com
Subject: Re: [PATCH net-next v2 1/2] net/ipv6: insert a f6i to a GC list only
 if the f6i is in a fib6_table tree.

On 12/8/23 14:43, David Ahern wrote:
> On 12/8/23 12:45 PM, thinker.li@...il.com wrote:
>> From: Kui-Feng Lee <thinker.li@...il.com>
>>
>> Check f6i->fib6_node and hlist_unhashed(&f6i->gc_link) before inserting a
>> f6i (fib6_info) to tb6_gc_hlist.
>>
>> The current implementation checks if f6i->fib6_table is not NULL to
>> determines if a f6i is on a tree, however it is not enough. When a f6i is
>> removed from a fib6_table, f6i->fib6_table is not reset. However, fib6_node
>> is always reset when a f6i is removed from a fib6_table and is set when a
>> f6i is added to a fib6_table. So, f6i->fib6_node is a reliable way to
>> determine if a f6i is on a tree.
> 
> Which is an indication that the table is not the right check but neither
> is the fib6_node. If expires is set on a route entry, add it to the
> gc_list; if expires is reset on a route entry, remove it from the
> gc_list. If the value of expires is changed while on the gc_list list,
> just update the expires value.
> 

I don't quite follow you.
If an entry is not on a tree, why do we still add the entry to the gc 
list? (This is the reason to check f6i->fib6_node.)

The changes in this patch rely on two indications, 1. if a f6i is on a 
tree, 2. if a f6i is on a gc list (described in the 3rd paragraph of
the comment log.)
An entry is added to a gc list only if it has expires and is not on the 
list yet. An entry is removed from a gc list only if it is on a gc list.
And, just like what you said earlier, it just updates the expires value
while an entry is already on a gc list.